The invention relates to a method for mass spectroscopic or mass spectrometric investigation of particles, preferably of isotopes or molecule ions, in which a particle beam is separated in a separating system in accordance with the different particle masses and the particles are detected in a detecting device, and in which in order to reduce detection errors caused by particles which possess a particle mass, especially adjacent mass, that deviates from the particle mass of interest during the (instantaneous) particle detection (abundance sensitivity), a correction takes place by using a braking potential to keep from the detecting device or to suppress (energy selection) particles having a (kinetic) energy that is smaller than that to be expected for the particles of correct mass to be detected, or particles having an energy loss of a predetermined value.
Moreover, the invention relates to a mass spectrometer, preferably for carrying out said method.
A mass spectrometer has a separating system by means of which a particle beam is separated in accordance with the different particle masses. In this process, the particle beam is normally fanned out into a plurality of discrete component beams. A sector magnet is normally a component of the separating system.
The relative mass distribution of particle masses inside the original particle beam can be determined by means of the mass spectrometer by detecting the particles of the component beams simultaneously or sequentially over a certain time interval. A detector element of the detecting device is tuned for this purpose to the particle beam to be recorded. Such a detector element can comprise, for example, an electron multiplier or also a Faraday cage.
The detection of particles of the component beams results in a mass spectrum having mass spectral lines. The possibility of distinguishing or separating individual spectral lines from one another during the analysis depends essentially upon the resolving power of the mass spectrometer.
Detection errors which are reflected in a correspondingly distorting fashion in the mass spectrum can result, inter alia, from scattering processes of the particles before entry into the detecting device. Due to such a scattering process, a particle can enter the detecting device at a location that does not correspond to the position of the component beam corresponding to the particle mass of the particle. This means that the detected particle is regarded as a particle of a mass that it does not really have at all. This erroneous detection thus leads to an enlargement of the area of a spectral line in the mass spectrum which does not correspond to the actual detected particle. In particular, due to such erroneous detections the spectral lines acquire so-called "tails" in their foot region. The spectral lines are thus widened in the foot region. In "tails" of strong spectral lines, in particular, weaker, adjacent spectral lines can vanish and thus remain unrecognised.
Since during scattering processes of the particles there is always a more or less large energy loss of the particles, the abovementioned "tails" are located essentially on the low-mass side of the spectral lines. However, "tails" can also arise on the high-mass side of the spectral lines if the energy loss of the scattered particles is relatively low.
For example, scattering processes can take place on residual gas molecules or also on surfaces. In this connection, the scattering processes on surfaces can lead to a relatively large scattering angle in conjunction with a relatively low energy loss of the particles, that is to say in particular to the "tails" on the high-mass side.
In a mass spectrometer or in a mass spectrometric method it is desirable, after all, to reduce erroneous detections of particles, and in this way to suppress the "tails" or spurs of the spectral lines.
Since the particles lose more or less energy during scattering processes, it is possible to sort scattered particles at least partially by means of an energy filter, that is to say to prevent them from entering the detecting device (energy selection). This can be done with the aid of a braking electrode upstream of the detecting device, by means of which a braking potential is built up against which all particles must run in order to reach the detecting device. In this process, the potential barrier of the braking potential can be tuned such that only unscattered particles can surmount said barrier, whereas scattered particles that no longer possess sufficient energy fail at the potential barrier and do not reach the detecting device. It is possible by means of said procedure at least to diminish the spurs of the mass spectral lines on the low-mass side.
As an example, all non-scattered particles could have an energy of approximately 10 keV. In this regard, there is a certain energy distribution of particles which depends upon the initial conditions in the particle source. In proportion to the mean energy of the particles, the width of the energy distribution or the "energy smear", amounts in this regard to 5.times.10.sup.-5, for example. In the example chosen, surge-induced energy losses are generally greater than 2 eV, so that it is possible to utilise an energy filter which can be tuned to filter out or retain all particles having an energy loss of between 50 eV and 1 eV. 1 eV is in the proportion of 1.times.10.sup.-4 to the chosen mean energy of 10 keV, so that even though it relatively reliably retains scattered particles a filter which filters out at this order of magnitude does not yet reach into the range of width of energy distribution of 5.times.10.sup.-5.
Although braking the particles to be detected can act to improve reduction of the low-mass spurs of the spectral lines, it has no such effect on the spurs on the high-mass side of the spectral lines. Scattered particles having only a slight energy loss pass through the filter.
Moreover, due to the energy filter all particles, that is to say also the unscattered particles, are braked at least, as a result of which the mass spectral lineshape is worsened over all, since the spectral line is widened hereby. This finally detracts from the resolving power of the mass spectrometer.